Conventional parking assistance systems have been known which measure the distance between a target parking position and a vehicle to detect the position of the vehicle relative to the parking position. Such systems are usually ones that utilize reflection of transmitted waves like a sonar and the like (see Japanese Patent No. 5137139, for example).
However, in the case of those systems that utilize reflection of transmitted waves, the area capable of receiving the reflected waves changes depending on the shape and orientation of the target object that reflects the transmitted waves. In addition, the transmitted waves might be reflected on an object other than the target object and cause false detection.